Since a polyolefin porous membrane exhibits excellent electrical insulation properties or ion permeability, it has been widely used as a separator in a battery or a capacitor or the like. In recent years, with the multifunctionalization and weight reduction of portable appliances, a high-power-density and high-capacity-density lithium-ion secondary battery has been used as an electric power source for the appliances. A polyolefin porous membrane is frequently used as a separator in such a high-power-density and high-capacity-density lithium-ion secondary battery.
Here, an organic solvent is usually used as the electrolyte in a lithium-ion secondary battery. Therefore, when an abnormal condition such as short-circuiting or overcharging happens to a lithium-ion secondary battery, the electrolyte can be decomposed to cause ignition in the worst case. In order to prevent such conditions, several safety features are incorporated in a lithium-ion secondary battery. Examples of the safety features may include the shutdown function of a separator.
The term “shutdown function” means that when a battery causes abnormal heat generation, the micropores of a separator are occluded by thermal fusion and the like to suppress the ionic conduction in an electrolyte and stop the progress of the electrochemical reaction. It is generally said that the lower the shutdown temperature, the higher the safety. Since polyethylene has an appropriate shutdown temperature, it is preferably used as a component of a separator.
However, the amount of heat generated when thermal runaway occurs may be large in a battery having a high energy. If the temperature continues to increase beyond the shutdown temperature, both electrodes are short-circuited by membrane breakage (hereinafter, also referred to as “short-circuit”) of a separator and further heat generation is likely to occur.
Under such circumstances, there is proposed a method in which a layer mainly composed of an insulating inorganic filler is formed between a separator and an electrode (Patent Documents 1, 2, 3, 4, 5, 6 and 7). In addition, these Patent Documents describe a method in which an inorganic filler layer is formed on a separator surface by applying a dispersion solution containing an inorganic filler and a resin binder on the separator surface which is a porous membrane.
Here, Patents Documents 1, 3 and 4 describe a method in which polyvinyl alcohol is used as a binder in a layer mainly composed of the insulating inorganic filler. In addition, Patent Documents 8, 9 and 10 describe a method in which polyvinyl alcohol is used as an adhesive for bonding between an electrode layer and a separator. Further, Patent Document 11 describes a method in which when a thin membrane composed only of an inorganic material is formed on a hole wall surface of a polyolefin porous membrane, adhesiveness of the hole wall surface is increased by surface treatment and thereby preventing peeling of the thin membrane composed only of an inorganic material.
Patent Document 1: Japanese Patent No. 3756815
Patent Document 2: Japanese Patent No. 3752913
Patent Document 3: Japanese Patent Laid-Open No. 2005-276503
Patent Document 4: Japanese Patent Laid-Open No. 2004-227972
Patent Document 5: Japanese Patent Laid-Open No. 2004-040499
Patent Document 6: Japanese Patent Laid-Open No. 11-080395
Patent Document 7: Japanese Patent Laid-Open No. 09-237622
Patent Document 8: Japanese Patent No. 3426253
Patent Document 9: Japanese Patent No. 3393145
Patent Document 10: WO99/31750
Patent Document 11: Japanese Patent No. 3797729